Thermal imaging systems that detect infrared radiation to produce an image capable of being visualized by the human eye are gaining widespread commercial use. These systems, which use a focal plane array (FPA) containing multiple thermal sensors, have been traditionally used in aerospace or military applications. In order for these systems to find success in the commercial market, reduction in their costs must be achieved.
A key component of thermal imaging systems is the window or dome used to protect the thermal sensors from the environment. These windows must be optically transparent in the 8 to 12 micrometer wavelength region to allow infrared (IR) energy to reach the thermal sensors. In order to achieve optical transparency, these IR windows are typically made of germanium, silicon, zinc sulfide, gallium arsenide, zinc selenide, and gallium phosphide, among other compounds. Windows made from these materials are typically very expensive.
Unfortunately, not only are windows made from those materials expensive, they are also prone to degradation or erosion when particles strike the window. Erosion of the window reduces its strength and ability to transmit infrared energy therethrough. This degradation can render the thermal sensors behind the window inoperable or even damaged should the window catastrophically fail.
Various methods have been previously developed to coat these IR windows. One previously developed solution to this problem involves placing protective coatings on the IR window. Because the protective coating must be transparent in the wavelength regions that the window operates, one type of protective coating has concentrated on traditional inorganic materials such as, for example, silicon, gallium phosphide, boron phosphide, diamond, germanium carbide, silicon nitride, silicon carbide, and oxides, to obtain the desired transparency. These coatings exhibit high strength, high fracture toughness, high hardness, and moderate to high elastic (Young's) modulus.
Another approach to protecting IR windows has been to use transparent polymer coatings that have low hardness and high strength on the surface of the windows. These polymers absorb and distribute the stresses of impacting particles, thereby protecting the underlying IR window. Such polymer coatings are generally inexpensive and readily available in films that can be placed on the exterior surface of an IR window. Unfortunately, these coatings are prone to delamination or peeling from the window's surface exposing the window to elements that may damage it. More importantly, these prior approaches that apply a protective coating to the IR window to protect the window require the traditional, generally expensive IR window. Including the traditional IR window with a protective coating in a thermal imaging system provides for a more durable system, but does not help reduce the system's costs. This prevents thermal imaging systems from achieving widespread commercial acceptance.
Yet another previous approach to building a low cost thermal imaging system has been to replace the traditional IR window with a window formed entirely from the IR transparent polymers. Unfortunately, the thickness required to achieve the desired strength and stiffness in a polymer window results in a window that presents a long optical path to incident IR energy. Significant amounts of the IR energy may be absorbed on this optical path through the polymer window preventing the IR energy from reaching the thermal sensors. This can prevent the thermal imaging system from providing the desired resolution during operation.